I have send emails repeatedly to the developers of DigID well over a year ago (Logius), and they have never even bothered to respond.
The app used to work - including scanning QR codes - around a year ago (without Google play services!). Eventually an update caused it to break. I avoided updating as long as possible but eventually the old version I had was deprecated and did no longer work because it was too old.
It's a shame that apps like this are dependent on third party services and that it doesn't work natively.
Nice to see fellow people from the Netherlands using GrapheneOS btw, we're one of few and I personally don't know anyone else that uses GOS.
Best of luck with getting through to Logius!